Traditional mixed agroforestry farms are regarded as sustainable agroecological systems contributing to agrobiodiversity conservation and household food and nutrition security in sub-Saharan Africa. However, in Kenya little is known on the level of agrobiodiversity of these mixed farms and its contribution to food and nutrition security. A case study was conducted to assess food plant and livestock diversity and to identify the biophysical and socioeconomic factors influencing food plant diversity in 30 smallholder farms in Western Kenya. The survey identified six livestock species and 59 food plant species. Higher food plant species richness was found on farms managed by wealthier households and older household heads. However, households with high on-farm food plant richness and diversity were not more food secure than households managing species-poor farms. The nonsignificant relationship between food security and agrobiodiversity during the time of this case study may have resulted from the fact that the surveyed 30 families sourced significant proportions of their food from markets and did not fully depend on their farms for food, particularly for spices and condiments, fruits, and animal source foods. Therefore, we suggest a diversification of farms through livestock and fruit tree farming for improving dietary diversity and incomes of the surveyed households.